Chapter 2. Novel Processes and Materials
Chapter 2. Novel Processes and Materials for
Infrared Nonlinear Optics
Academic and Research Staff
Professor Peter. A. Wolff, Dr. Sunny Y. Auyang
Graduate Student
David B. Walrod
2.1
Project Description
Sponsor
National Science Foundation
Grant EET 87-18417
Optical systems such as signal processors,
limiters, spatial light modulators, and optical
computers require large and fast optical nonAlthough free carrier-induced
linearities.
nonlinearities in semiconductors have the
picosecond speeds required for use in many
applications, they were considered too weak
to be practical for optical systems applications. In our work, we have demonstrated
that these nonlinearities can be substantially
enhanced in certain materials and in structures such as superlattices. We have recently
observed the first optical nonlinearity caused
by subband effects in InSb/AllnSb and
HgTe/CdTe superlattices.
Superlattices have the potential for generating large optical nonlinearities because
their band structures can be tailored to optimize particular nonlinear effects.
Several
novel nonlinear optical processes that utilize
either the large subband nonparabolicity in
the growth direction' or intersubband transitions in various configurations 2 specific to
superlattices have been proposed. To our
knowledge, these processes have not been
observed.
With support from the National Science
Foundation, we have observed optical nonlinearities caused by subband nonparabolicity
in InSb/InAISb and HgTe/HgCdTe superlattices by using four-wave mixing techniques
with CO 2 lasers.
We have observed an
enhancement in the third-order susceptibility,
X~(3)when the laser light was polarized along
the growth direction instead of in-plane.
This experiment demonstrated that the superlattice subband structure induced larger
optical nonlinearity than that of its bulk
counterpart. 3
The coupling between light and electrons is
given by p.E, where E is the electric field of
the light and p the momentum of the nth
energy subband S,(p). Therefore, to observe
nonlinear optical effects caused by subband
structures peculiar to superlattices, the electric field of the light must be polarized in the
superlattice growth direction. We designated
the direction perpendicular to the plane of
the superlattice as z and the in-plane direction as x.
Light polarized along x probes
only subband structures parallel to the superlattice plane, en(px), which is not different
from their bulk counterparts. The interesting
superlattice effects are contained in Es(pz);
large enhancement of pz-nonparabolicity has
been predicted. Furthermore, intersubband
transitions are induced by Ez but not Ex.
1 W.L. Bloss and L. Friedman, Appl. Phys. Lett. 41:1023 (1982); S.Y. Yuen, Appl. Phys. Lett. 43:813 (1983); G.
Cooperman, L. Friedman, and W.L. Bloss, Appl. Phys. Lett. 44:977 (1984); Y. Chang, J. Appl. Phys. 58:499
(1985).
2 S.Y. Yuen, App. Phys. Lett. 43:813 (1983); D.J. Newson and A. Kurobe, AppL Phys. Lett. 51:1670 (1987).
3 D. Walrod, S.Y. Auyang, and P.A. Wolff, App. Phys. Lett. 56:218 (1989).
103
Chapter 2. Novel Processes and Materials
parabolicity of the lowest subband eo(pz),
were nonresonant, not placing as severe a
demand on the specification of the sample.
However, the enhancement over the bulk
was smaller.5
Superloti ce
Substrate \
We observed optical nonlinearities caused by
subband structures in the growth direction in
an AllnSb/InSb superlattice by using an endfiring technique. The AIInSb/InSb superlattice we studied consisted of 50 periods, each
with 45 A of InSb and 60 A of Alo.o
08 ln 92 Sb.
This superlattice was separated from the 350
um Cr-doped GaAs substrate by a 200 A
The samples were
AISb buffer layer.
undoped. We used a 50 ym InSb epilayer
grown under the same conditions on the
same kind of substrate for a control sample.
(b)
(a)
k
(c)
(d)
Figure 1.
The coupling to en(pz) is difficult to achieve in
conventional nonlinear optical experiments in
which light impinges upon the surface of the
Because of the large
sample (figure 1).
indices of refraction of semiconductors Ez
inside the sample, there was never more than
a small fraction of the total field. Even at
Brewster angle incidence, less than ten
percent of the light intensity was polarized
along z since the nonlinear signal is proportional to the cube of the input intensity.
Therefore, the third-order nonlinear susceptibility X(3) due to en(Pz) must be at least fifty
times larger than the bulk susceptibility to be
distinguished in this geometry. We predicted
these large susceptibilities for intersubband
transitions when the subband gap was
exactly matched to the photon energy.4
Intersubband transition was a resonant
effect, however, and the predicted
3
;( )
might
not be achievable if the actual subband separation failed to match the photon energy.
Optical nonlinearities generated by the non-
For the end-firing experiments, we cleaved
off a strip about 1 mm in width. The cleaved
We
edge provided good transmission.
observed about 50 percent transmission
through the 1-mm sample; almost all loss
was due to reflection. To ensure that the
GaAs substrate did not contribute to the
nonlinear optical signal, we cut off small
sections of the samples and polished off the
Then we compared the
superlattice.
substrate with a sample in which the superlattice was left intact. In all cases, the nonlinear signal vanished when we removed the
superlattice.
The end-firing configuration enabled us to
probe the z-direction dispersion relation by
The
changing the polarization direction.
four-wave signal from the InSb epilayer does
not change as the light polarization is rotated
90 degrees. This result suggests that the
material is isotropic. The four-wave signal
from the superlattice was comparable to that
of the InSb epilayer when the light was
However, as the
polarized along x.
polarization was rotated to the i direction,
by
increases
signal
four-wave
the
twenty-two times, as depicted in figure 2.
4 S.Y. Yuen, Appl. Phys. Lett. 43:813 (1983); D.J. Newson and A. Kurobe, Appl. Phys. Lett. 51:1670 (1987).
5
W.L. Bloss and L. Friedman, Appl. Phys. Lett. 41:1023 (1982); S.Y. Yuen, Appl. Phys. Lett. 43:813 (1983); G.
Cooperman, L. Friedman, and W.L. Bloss, Appl Phys. Lett. 44:977 (1984); Y. Chang, J. Appl Phys. 58:499
(1985).
104
RLE Progress Report Number 132
Chapter 2. Novel Processes and Materials
By comparing the optical nonlinearities generated by different light polarizations, we
ascertained that the z-direction subband
structure gave an enhancement of X(3) over
A
Z
0x0
2.0o
0
co
03
o
uo
O
L=
the bulk value. There are several possible
explanations for the enhancement, but we
them
distinguish
to
unable
were
quantitatively. Our research is continuing in
this direction.
Publications
Auyang, S.Y., and P.A. Wolff. "Free-CarrierInduced Third Order Optical Nonlinearities
in Semiconductors." J. Opt. Soc. Am. A
6:2696 (1989).
Walrod, D., S.Y. Yuen, P.A. Wolff, and W.
"Optical Nonlinearities due to
Tsang.
Subband Structures in Alo08 ln0.92Sb/InSb
56
Appl. Phys. Lett.
Superlattices."
(3):218-220 (1989).
Figure 2.
105
106
RLE Progress Report Number 132